Autofrettage is as such a known metal fabrication technique in which a pressure vessel is subjected to enormous pressure, causing internal portions of the part to yield and resulting in internal compressive residual stresses. The aim of autofrettage is to increase the fatigue durability of the final product under cyclic load. The technique is commonly used in manufacturing high-pressure pump cylinders, warship and tank gun barrels, and fuel injection systems for diesel engines. While some work hardening will occur, that is not the primary mechanism of strengthening.
Prior art document published US 2010/0154501 A1 is directed to an autofrettage process of a cylindrical thick-walled work piece used in “common rail” diesel injection systems for combustion engines. The purpose of this treatment is to increase the fatigue behavior of the work piece. The work piece is specifically cylindrical without threaded ports. The piece is positioned and pressed between two elements of the autofrettage tooling. The bottom element of the tooling comprises sealing means coming into a contact with a flat front surface of the work piece whereas the upper element equipped with a displacement piston generating the autofrettage pressure comes into a contact with the opposite front surface of the work piece by means of a conical surface. The work piece is therefore pressed between the bottom and upper element so as to have a tight cooperation.
Prior art document published U.S. Pat. No. 4,417,459 discloses similarly to the above document an autofrettage process for thick-walled cylindrical work pieces. In this teaching, the autofrettage process comprises a cyclic treatment where the pressure is applied in a repetitive and cyclic manner. Similarly to the above document, the work piece is sandwiched between two elements of the autofrettage tooling so as to provide a tight connection with the two opposed connecting ports of the work piece.
Prior art document DE 196 50 736 C1 discloses an autofrettage process similar to the process of the two above mentioned documents.
The above mentioned teachings do not however address the autofrettage of work pieces comprising threaded connection port, like the body of gas valve. Threads constitute also weak points with regard to fatigue resistance.
For hydrogen-powered vehicle applications, the COMMISSION REGULATION (EU) No 406/2010 of 26 Apr. 2010 and implementing Regulation (EC) No 79/2009 of the European Parliament and of the Council on type-approval of hydrogen-powered motor vehicles, have provisions that the components of the hydrogen supply of the vehicles must satisfy different tests. The valves must among others satisfy cyclic pressure tests. It is therefore wished that the valves for such applications can withstand a maximum static test pressure of 105 MPa (1050 bar) and a cyclic hydraulic pressure test of N=150 000 cycles with a cyclic pressure loading of 87.5 MPa and a stress ratio R=min stress/max stress=0.